The present invention relates to process devices of the type used in industrial processes. More particularly, the present invention relates to devices such as transmitters which control electrical current in a process control loop.
Field devices such as process controllers, monitors and transmitters, are used in the process control industry to remotely control, monitor or sense a process variable. For example, a process variable may be transmitted to a control room by a transmitter for use in controlling the process or for providing information about process operation to a controller. For example, information related to pressure of process fluid may be transmitted to a control room and used to control the process, such as oil refining.
One typical prior art technique for transmitting information involves controlling the amount of current flowing through a process control loop. Current is supplied from a current source at a location such as in the control room, and the transmitter controls the current flowing in the loop from its remote location in the field. For example, a 4 mA signal can be used to indicate a zero reading and a 20 mA signal can be used to indicate a full scale reading. More recently, transmitters have employed digital circuitry which communicates with the control room using a digital signal which is superimposed onto the analog current signal flowing through the process control loop. One example of such a technique is the HART® communication protocol developed by Rosemount Inc. The HART® protocol and other such protocols typically include a set of commands or instructions which can be sent to the transmitter to elicit a desired response, such as transmitter control or interrogation.
When an analog loop current is used to represent information such as a process variable, the accuracy at which the loop current can be set can be a limiting factor on the accuracy of the transmitted process variable. The controlled loop current may be susceptible to drifting. In other words, the controlled loop current can change value over time as electrical components that control the current tend to age.